In the prior art, a steel sheet subjected to chromate treatment by using treatment liquid including as main components chromic acid, dichromic acid, or salts thereof has been widely used as a steel sheet for consumer electrical appliances, a steel sheet for construction materials and a steel sheet for automobiles in order to improve corrosion resistance (white rust resistance and/or red rust resistance) of a surface of the zinc or zinc alloy-coated steel sheet. The chromate treatment is a relatively simple and economical method for obtaining steel sheets having good corrosion resistance.
Such chromate treatment as described above uses hexavalent chromium as a substance of which use is to be strictly controlled to prevent pollution. In the chromate treatment, hexavalent chromium is handled in a closed system in which hexavalent chromium is completely reduced and retrieved so that hexavalent chromium is not released into nature. Further, the chromium dissolution from a chromate film can be reduced to substantially zero by a sealing effect of an organic film. Accordingly, there is substantially no possibility that the environment and human bodies are polluted by hexavalent chromium. However, in view of the recent global environmental problems, a tendency toward self-imposed restraints on use of hexavalent chromium has become active. In addition, for protecting the environment from pollution caused by dumped shredder dust of wastes, movement of reducing hexavalent chromium in products as best as possible, to possibly eliminate it, has already begun.
Given that situation, many proposals of so-called chromium-free technology with no chromate treatment have been made for preventing white rust formation on zinc or zinc alloy-coated steel sheets using, for example, an inorganic compound, an organic compound, an organic polymer material, or a composition containing a combination of these materials. For example, such a technology includes a technique of forming a surface treatment film on zinc or zinc alloy-coated steel sheets. Various effective techniques have been proposed so far; however, new problems to be solved, which have not been recognized up to this time have been found as such chromium-free technology becomes more popular.
A first object is to improve corrosion resistance at worked portions in zinc or zinc alloy-coated steel sheets. Surface-treated zinc or zinc alloy-coated steel sheets are made into product after working processes (cutting, bending, or welding of parts). When surface-treated zinc or zinc alloy-coated steel sheets are bent, the coating on a surface area of the bent portions is stretched. That is, the bending makes the surface treatment film being stretched. As a result, the surface treatment film is damaged to expose the zinc or zinc-coated surface, and corrosion resistance of the exposed portion is deteriorated, which is a problem. In particular, bending causes continuous damage on a film and coating unlike local damage caused by an extrusion process. Therefore, it is significantly difficult to make such worked (processed) portions have satisfactory corrosion resistance.
A second object is to ensure solvent resistance of a surface treatment film. In the above working processes, oil stain attached on or symbols written with a marker on a film surface are sometimes wiped off with solvents, and in such cases, a surface treatment film has often come off or turned into whitish color (bleached) due to the solvents. When the surface treatment film comes off, sufficient corrosion resistance of zinc or zinc alloy-coated steel sheets cannot be obtained, and appearance quality decreases when the surface treatment film is bleached.
A third object is to ensure coating properties of a surface treatment film. A surface of surface-treat zinc or zinc alloy-coated steel sheets processed as described above is sometimes coated after being washed with an alkaline cleaner or the like (alkaline-degreased) to clean the surface. Therefore, topcoat coating properties after such alkaline degreasing might be required; however, discussion about these properties has not yet been found in any prior documents.
A fourth object is to ensure blackening resistance. When zinc or zinc alloy-coated steel sheets are exposed to an environment at high moisture or high temperature for a long time, blacking likely occurs which blackens the coating surfaces. Such blackening becomes prominent when the coating layer includes an element such as Mg or Al in particular; thus, a surface treatment film on zinc or zinc alloy-coated steel sheets is required to be excellent in blackening resistance.
A fifth object is to ensure sweat resistance of a surface treatment film. In addition to the above blackening, when a person directly touches a zinc or zinc alloy-coated steel sheet containing an element such as Mg or Al, the portion touched is observed to turn into black over time. This is a phenomenon in which human sweat which is weekly acidic adheres to a steel sheet to blacken the steel sheet surface. Accordingly, a surface treatment film on a zinc or zinc alloy-coated steel sheet is also required not to turn into black even when sweat adheres to it, in other words, to be excellent in sweat resistance.
As described above, zinc or zinc alloy-coated steel sheets treated by surface treatment achieving various excellent characteristics are demanded. Concrete examples of conventional chromium-free techniques include the following.
JP S53-121034 discloses a method of forming a film by applying an aqueous solution containing water-dispersible silica, an alkyd resin, and a trialkoxysilane compound on a metal surface and drying it.
JP S57-44751 discloses a surface-treatment method aimed at imparting corrosion resistance to a metal material with the use of a water-soluble resin made of hydroxypyrone compound derivatives. Further, JP H1-177380 discloses a method of imparting corrosion resistance to a metal material with the use of an aqueous solution of a hydroxystyrene compound or a water-dispersible polymer.
JP H11-310757 discloses a technique using a surface treatment agent containing a water-based resin, colloidal silica, and ammonium vanadate at certain proportions.
However, any of the foregoing techniques have not developed a film imparting satisfactory corrosion resistance which can replace chromate films.
As another example, JP 2000-248369 discloses a technique of forming a surface treatment film containing an organic resin and a thiocarbonyl group-containing compound; however, sufficient corrosion resistance cannot be achieved after alkaline degreasing.
JP H11-58599 discloses a technique of treating a metal sheet surface with a treatment agent in which an aqueous solution of lithium silicate contains an organic resin, a silane coupling agent, and a solid lubricant. However, inorganic components easily form hard polymers, which would cause poor corrosion resistance at portions processed by bending or the like. Further, since the treatment agent contains an alkali metal, the secondary paint adhesion would not be enough.
JP 2006-43913 discloses a technique of forming a resin film with the use of an aqueous solution of resin which contains a carboxyl group-containing polyurethane resin, aqueous dispersion of an ethylene-unsaturated carboxylate copolymer, silica particles, and a silane coupling agent at certain proportions; however, the solvent resistance or the corrosion resistance of the processed portions are not enough.
JP 3573307 discloses steel sheets having a film containing a urethane-based resin, a lubricant, an inorganic colloid compound, and a silane coupling agent at certain proportions; however, the steel sheets are designed for electrodeposition coating, and the corrosion resistance at processed portions is not enough although they are excellent in electrodeposition coating properties.
JP 2001-59184 discloses a surface treatment liquid in which a silane coupling agent and a urethane resin are mixed to achieve a pH of 2.5 to 4.5; however, the corrosion resistance after alkaline degreasing is poor, and the solvent resistance is not enough either.
JP 2003-155451 discloses a technique of forming a film using a treatment agent containing an aqueous dispersion resin, silica particles, and organic titanate at certain proportions; however, the corrosion resistance at the processed portions is not enough.
JP 2006-82365 and JP 2004-238716 discloses a technique of forming a film using a treatment agent containing a certain water-based epoxy resin dispersion, a urethane resin dispersion, a silane coupling agent, a phosphoric acid and/or a phosphate compound, and a compound having 1 to 5 fluorine atoms in one molecule; however, some shortage of alkali resistance leaves room for improving corrosion resistance and coating properties after alkaline degreasing. Besides, there is room for improving corrosion resistance at the processed portions and solvent resistance.
JP 2001-181860 discloses a technique of forming a film with the use of a treatment agent containing a specific resin compound, a vanadium compound, and a certain metal; however, since alkali resistance is not enough, satisfactory corrosion resistance after alkaline degreasing is not obtained. Further, problems of tendency to yellowing under heating or the like have not been solved.
JP 3883831 discloses a technique of forming a film with the uses of a processing agent containing a specific resin compound, a cationic urethane resin having a cationic functional group, a silane coupling agent having a reactive functional group, a Ti compound, and an acid compound at certain proportions. According to this technique, a film excellent in corrosion resistance and fingerprint resistance can be obtained; however, corrosion resistance after alkaline degreasing, corrosion resistance at the processed portions, and solvent resistance are not discussed to leave room for such resistances to be improved.
JP 4078044 discloses a technique involving a surface treatment agent containing at least one type of water-based resin selected from cationic and nonionic urethane resins, a specific resin compound, a metal compound including a specific metal, and water. However, whichever of alkali resistance, corrosion resistance at the processed portions, and solvent resistance is not discussed, and these characteristics are not obtained sufficiently.
JP 2006-152436 discloses a technique of using a surface treatment agent containing cationic urethane, a cationic phenol-based polycondensate, titanium, and a compound containing a specific metal at certain proportions. However, no discussion about solvent resistance and coating properties has been made therein, and these characteristics were not satisfactory.